There are some news about the MAME port to iPhone (there is no download yet).

Quoted directly from the homepage:

CURRENT STATUS: As expected, performance is an issue. For example, Pacman and Q*bert run quite well, but Galaga and Gyruss are marginal even with frame skipping and lowered sound quality. The SDL port was the quickest way to get everything up and running without having to delve into the guts of MAME. I just started working on porting older versions and hopefully I’ll find something that’s acceptable.

http://code.google.com/p/iphone-sdl-mame/